Diet, schmiet. Soft-drink makers are racing to replace or play lown the word "diet" in brand names in favor of alternative terms that they hope will help fatten sales.

The Coca-Cola Co. kicked off the trend in September by renaming the diet version of its Sprite lemon-lime soda Diet Sprite Zero. That name will probably be changed before long to Sprite Zero, as the soda is called in more than 20 markets overseas.

The Pepsi-Cola Co. division of PepsiCo. last month changed the name of the diet version of its lemon-lime soda, Sierra Mist, to Sierra Mist Free. Both Sierra Mist Free and Diet Sprite Zero are being promoted in multimillion-dollar campaigns on television, in print, on posters and in stores.

The rebrandings come as marketers are struggling to inject some effervescence into sales of carbonated soft drinks, which have been losing luster— particularly among younger consumers — to products like water, tea and energy beverages. About the only sodas gaining sales are those without sugar, so the renamings are intended to capitalize on that momentum.

"Diet and sugar-free soft drinks are what's driving all the growth in carbonated soft drinks right now," said Steve Sears, vice president for marketing for flavored soft drinks at the Pepsi-Cola North America division of Pepsi-Cola in Purchase, N.Y. "This is one opportunity we saw for Sierra Mist."

The terms "free" and "zero" are intended mainly to help update perceptions of sugarlesc soft drinks, because research shows the word "diet" — which emerged in the '60s and '70s as a more modern way to say "dietetic" — can carry unwanted baggage, particularly among younger men.

"There's a language of 'diet' that's very unappealing," said Rony Zibara, executive creative director for North America at FutureBrand in New York, an agency owned by the Interpublic Group of Cos. that specializes in corporate and brand identities.

"The cues and signs of 'diet' say 'dated,' " Zibara said. "No one wants to be seen walking down the street with a diet beverage in hand."

In addition to evoking a bygone era replete with brands like Metrecal, Patio Diet Cola, Sugar Twin and Tab, the word "diet" can have unpleasant connotations, said John Diefenbach, a partner at TrueBrand in San Francisco, also a corporate and brand identity consultant.

"It's a word that represents somepling that doesn't taste good, a punishment, and people don't want to be punished," Diefenbach said. "They want something that tastes good."

Still, there are risks to eliminating a word like "diet" from familiar brands.

"I have doubts about renaming products that consumers already know," said John D. Sicher, editor and publisher of Beverage Digest, an industry newsletter based in Bedford Hills, N.Y. "I'm not sure it's a great idea." The risk is that products may lose whatever competitive advantage they now enjoy in an overcrowded category.

Even so, "using names other than 'diet' makes sense on new products," Sicher said, "as a way to broaden the segment."

Diet Sierra Mist was introduced nationally early last year, along with Sierra Mist; Diet Sprite has been around for three decades. The marketers behind both brands said their goal was to respond to changing consumer tastes.

"What we found in the new name is that it appeals to .onusers of Diet Sprite," whether those consumers had stopped buying Diet Sprite or had never tried it at all, said Dan Dillon, vice president for marketing in the diet unit of Coca-Cola North America.

"And 'zero' is a much better, more accurate description of the product," Dillon said, because it extends beyond "zero calories, zero carbs, zero sugar" to encompass "zero color and zero caffeine."

Sears at Pepsi-Cola North America said "Taking off 'diet' and putting 'free,' a great, wonderful word, at the end lets us lead off with 'Sierra Mist' in the name," Sears said.

Pepsi-Cola North America tried something similar a couple of decades ago, rebranding the caffeine-free versions of Pepsi-Cola and Diet Pepsi-Cola as Pepsi Free and Diet Pepsi Free. The renamings even served to set up a joke in the 1985 movie "Back to the Future," when Marty McFly (Michael J. Fox), visiting 1955, is told by a counterman after he orders a Pepsi Free, "You want a Pepsi, pal, you're gonna pay for it."

Sears said the word "free" was more suitable now than then, when the company was "taking one attribute, caffeine, and trying to build a case around it, compared with this product, where everything about it we're able to call 'free.' "

While it is too early to gauge sales for Sierra Mist Free, Sears said, in research the prospects for a rebranded product "exceeded our expectations." At Coca-Cola North Amesica, Dillon said, "we've seen an impact after the name change" to Diet Sprite Zero in terms of improved sales, citing company policy in declining to provide details.

In a speech last week at a conference sponsored by Beverage Digest, E. Neville Isdell, the new chairman and chief executive of Coca-Cola, said that in countries where Diet Sprite was being sold as Sprite Zero, the brand was "growing in the high double digits, versus single-digit growth for Diet Sprite."

Isdell praised the Sprite Zero brand name, which first appeared in 2002, in Greece, for "focusing on functional benefits" like no sugar or caffeine "with a name that appealed more broadly than 'diet' or 'light.' " (The word "light" commonly replaces "diet" in the names of sugarless sodas outside the United States because fewer consumers overseas follow diet or weight-loss regimens.)

In the Beverage Digest newsletter last week, Sicher reported on plans by Coca-Cola North America to further expand its sugar-free soft-drink offerings next year. He had obtained materials, he wrote, suggesting that the company is testing a product named Coke Zero.

Dillon, when asked for comment, replied: "There will definitely be news in 2005. There's an opportunity in the diet category and we're leveraging the consumer trends." In the meantime, he said, the campaign for Diet Sprite Zero will continue into the new year; it carries the theme "Nothing is lighter than Zero" and is being created by Ogilvy & Mather Worldwide in New York, owned by the WPP Group, which also creates ads for Sprite.

As for Sierra Mist Free auguring a renaming for other Pepsi-Cola North America diet brands, Sears said: "Sierra Mist is a new brand, and diet is a relatively small part of its sales. Renaming could create much more confusion for larger brands" like Diet Pepsi or Diet Mountain Dew.

BBDO Worldwide in New York, part of the Omnicom Group, is creating the campaign for Sierra Mist Free, which carries the theme "Drink freely." The television commercials feature the comedians Fred Willard and Michael McKean in confused debate, reminiscent of the scene in "Back to the Future," over the meaning of the word "free."